Capactive, magnetic and ferroelectric memories are extensively used to store data coded in binary form. In capacitive type memories, each information element or bit is stored in the memory as an electric charge Q. In certain types of capacitive matrix memories, binary values of one and zero are stored as the presence and absence of electric charges on capacitors. In other types of capacitive matrix memories, binary zero and one values are indicated by remnant capacitive and magnetic values, respectively associated with minus and plus charges and minus and plus magnetic fluxes.
Capacitive memories wherein binary zero and one values are represented by the presence and absence of electric charges are generally produced in metal oxide semiconductor (MOS) technology or metal nitride oxide semiconductor (MNOS) technology. A memory produced by MOS technology has access times of 450-1200 milliseconds and erase times of the order of one microsecond, whereas a memory produced by MNOS technology has access times of one microsecond and erase times of the order of 1-100 milliseconds. The significant difference between MOS and MNOS capacitive memories is that a dynamic MOS memory, i.e., an MOS memory that can be changed, is capable of retaining data in storage for only approximately one millisecond, whereas a dynamic MNOS capacitive memory can retain data for years.
Both MOS and MNOS memories, as well as ferroelectric and magnetic memories, have the disadvantage of being affected by a number of external physical disturbances. In particular, MOS and MNOS capacitive memories are frequently altered by exposure to certain types of electromagnetic radiation, such as ultraviolet radiation, that alters the amount of charge distributed within the memory. The charge alteration may modify the values of the stored data. Similarly, repeated access to MOS and MNOS capacitive memories causes progressive reduction in the quantity of charge stored in the memories. Also, variations in the current and voltage supplies for the memories may disturb and alter the stored data. MOS and MNOS memories also may prematurely age, to affect the permanence of data stored thereon. Magnetic and ferroelectric memories are also subject to variations, for example, in response to ambient magnetic and electric fields. These difficiencies are deleterious to the integrity and reliability of data stored in the various types of memories.
Several structures have been employed in the past in an attempt to maintain the integrity of data stored in memory matrices. For example, shields for magnetic, electric and electromagnetic fields have been provided about the periphery of the memory matrices to prevent the fields from being coupled to the matrices. Also, circuitry has been employed to establish regeneration cycles for charges stored in MOS and MNOS memory matrices. Circuitry has also been employed to decouple the memory matrices from drive circuits therefor in the event of failure of the drive circuits. Despite the use of these various structures to prevent variations in the values stored in a memory matrix, one can never be certain that a memory matrix has not been exposed to radiation at a particular instant, or that an electric phenomenon has not occurred which alters the data stored.
It is, accordingly, an object of the present invention to provide a dynamic memory that is constructed to enable undesirable changes in the memory elements to be detected, and to a method of making same.
It is another object of the invention to provide a dynamic memory with a checking device to determine whether or not a change in the memory is caused by means other than the circuits which normally drive the memory, or by secondary effects which cause undesirable changes, and to a method of making same.
Another object of the invention is to provide a memory with a circuit that enables the memory to be checked, and to assure effective decoupling of the checking circuit from the memory when the memory is in normal use, i.e., responsive to data read and write commands.